1. Field of the Invention
The present invention is directed to NMR imaging techniques for imaging paramagnetic species inside a sample and for determining information on the electrical activity of the brain of a subject.
2. Description of the Prior Art
Electron spin resonance (ESR) has been used to measure the concentration of paramagnetic species in samples for many years. As disclosed in U.S. Pat. No. 3,090,003, the ESR technique uses microwave frequency signals transmitted to a resonator cavity within which is placed the sample. The cavity is within a homogeneous magnetic field. The measurement of the concentration of paramagnetic species, such as radicals, transition metal atoms, excited molecules, etc., inside a man or a large animal has long been regarded as impossible because of the difficulty in constructing a cavity that would encompass the large volume of tissue, particularly when dealing with a living person.
In addition, imaging of paramagnetic species using the ESR technique is in a much earlier stage of development than NMR. For example, spatial distributions of paramagnetic species have been measured by a spectrometer using ESR techniques in U.S. Pat. No. 4,280,096. Gradient coils similar to those used in NMR are used to produce one-dimensional images. NMR and ESR techniques have been combined to record and observe spectra in electron nuclear double resonance (ENDOR). An ESR spectrometer is adapted to resonate nuclear spins by adding a radio frequency oscillator to the system. Examples of the ENDOR technique can be seen in U.S. Pat. Nos. 3,532,965 and 3,250,985.
In the prior art gyromagnetic systems, paramagnetic species are not detected directly by means of their electron resonance but by their effect on the relaxation times of nuclei undergoing nuclear spin resonance. The direct observation of ESR in large objects has been frustrated because of difficulties in designing a resonator with sufficiently high quality factor (Q) of dimensions many times larger than the operating wavelength.
Recently, Holder proposed to use ESR at about 1 GHZ to obtain images of the human head, using a technique derived from NMR imaging; referring to Holder, The Potential Use of ESR or Impedance Measurement to Image Neuronal Activity in the Human Brain, Electric and Magnetic Fields in Medicine and Biology, Conference Proceedings, London, 1985, IEEE Conference Publication 257. Localization of the neural activity of the brain is at present possible only by means of electroencephalography, but the precision of localization is rather low, at most of an order of 1-2 cm. The deeper structures in the brain are even less accessible and the localization rapidly deteriorates. Furthermore, localization by means of EEG lacks a reference grid or an anatomical reference to known parts of the brain. At this time, most accurate in localization of averaged metabolic activity is Positron Emitter Imaging, but it is by no means certain that the metabolic map will correspond to the electric activity map.
However, from the viewpoint of neurology, even more useful information can be obtained from the reconstructed images of the electrical firing of nerve cells in the human brain, since this would allow analysis of functional activity of neuroatomical pathways which cannot be achieved with the current techniques. Holder investigated the possibility that neuronal firing can be detected by a form of electromagnetic radiation which can then be reconstructed to form three-dimensional images of this functional activity. Holder teaches the use of ESR and impedance imaging as giving the best results. Holder states that NMR, being well established for spectroscopy and imaging, could be employed to detect neuronal firing, but that current flux from ions moving across the neuronal membrane would be too small to be detectable by NMR.